Familiar Letters of the Physics of the Earth

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General Books LLC, 2009 - Literary Collections - 136 pages
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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1851 Excerpt: ...from neighbouring rivers or brooks, have a temperature varying with the seasons, or even from week to week, so that their annual mean cannot differ very far from that of the ground. LETTER IX. ON THE TEMPERATURE OF THE LOWEST STRATUM OF THE ATMOSPHERE. The mean temperature of the lowest stratum of air that rests on the ground is generally the same as that of the soil, however great the variations may be, which you observe from time to time. It was formerly believed that this agreement occurred only in temperate latitudes, and that in the torrid zone the temperature of the ground was lower, and, on the other hand, that towards tbe polar circles, and beyond them it was higher than that of the air. This assumption was grounded on observations which had been made on springs. But the inference has lost its force since we learned that springs frequently receive their supplies in great part from considerable distances, thus bringing heat from the depths below, and cold from the high lands above. It must then at present be considered as certain, that there is this relation between the temperatures of the air and of the soil, that their yearly mean values are the same. The direct determination of the mean temperature of the air is attended with greater difficulties MANNER OF OBSERVATION. 133 than of the ground. The cause of this lies in the slight density of the air, and in the increased effect which this gives to any heat-influences acting simultaneously with the sun upon the thermometer. A thermometer of extreme accuracy, but rather sluggish in its movements, should be suspended in the shade, if possible on the north side of a building, at a distance of one foot from the wall; and its position should always be such that it is neither exposed directly or indirectly...

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